A general model for the study and optimization of reversed cycle thermal machines with two heat reservoirs is presented. The model is based oil the First and Second Laws or Thermodynamics, heat transfer equations at finite thermal source and sink capacities, and entropy generation terms in order to consider the internal and external irreversibilities of the cycle. The proposed model is applied to real-operating machines for which several constraints are imposed. Some results generated by the model when applied to refrigeration machines are presented. They point out interesting results regarding the limits of the variation range for the model variables Under different operating conditions and also for different variation laws for the internal entropy generation term. Sensitivity Studies with respect to the evaporator heat transfer area and other decision parameters show the dependence of the refrigerating machines performances on the model variables and decision parameters and what the investment concern is with respect to the gain in performances. Experimental data are Used in this study, in comparison with the analytical ones. Copyright (C) 2008 John Wiley & Sons, Ltd.